In magnetic resonance imaging (MRI), signals obtained from moving tissues (e.g., flowing blood) may be contaminated with signals from static tissues (e.g., vessel wall), in an example of partial volume effects. This may be particularly challenging where spatial resolution is low. MRI has been used for oximetry, but partial volume effects may affect the accuracy of the measurement.
The use of oximetry is a well understood and recognized technique for the measurement of blood oxygen saturation levels. However, conventional non-invasive oximetry methods may not be suitable in some cases, for example in premature babies with patent ductus arteriosus (PDA). PDA is a heart problem that occurs soon after birth in some babies. In PDA, abnormal blood flow occurs between two of the major arteries connected to the heart as a consequence of the blood vessel (called the ductus arteriosus), normally a component of fetal blood circulation, not closing shortly (typically hours to days) after the baby's birth. In such cases, there is typically a need to use cardiac catheterization, a more invasive form of intermittent blood sample, to determine arterial blood gases. The invasive nature of this pulmonary artery oxygen measurement may be stressful for the patient, for example an already fragile infant. Non-invasive techniques may be erroneous or may provide only indirect and/or very rough estimates of blood oxygen levels in such patients (e.g., limited to peripheral vessels).
MRI, a technique for non-invasive imaging of adults and children, has been used for non-invasive measurement of oxygen saturation of blood. For example, U.S. Pat. No. 5,233,991, the entirety of which is hereby incorporated by reference, describes a method and apparatus for estimating blood oxygen saturation using magnetic resonance (MR). This MR oximetry may be based on a quantification of T2 relaxation time. However, this conventional MR oximetry technique may be unsuitable for accurate determination of oxygen saturation in relatively small blood vessels (e.g., vessels in infants and children). As such, this technique has seen relatively little application to use in neonatal infant care.
There is an inherent MR tradeoff between spatial resolution and signal-to-noise ratio. With insufficient spatial resolution (e.g., as in relatively small blood vessels), T2 measurements may become biased by partial volume effects (e.g., where the image pixel or voxel is so large relative to the vessel cross-section that the vessel occupies only part of the pixel or voxel). This may be overcome by increasing the spatial resolution (e.g., using smaller image pixels or voxels), but this typically leads to a decrease in signal-to-noise of the measurement and, as a result, may produce less accurate T2 values. Increasing the spatial resolution may also require a longer time for image acquisition, which may be unsuitable for very young patients or any other patients who are unable to hold still for a lengthy period of time.